How to Interpret an ECG in Seven Steps

Physicians encounter ECGs in their clinical routine every day. Additionally, ECGs are frequently the topic of exams, which is reason enough for us to provide an analysis algorithm that will aid students in interpreting an ECG. Learn the seven steps to interpret an ECG and test your knowledge by taking the ECG quiz.

Sinus arrest with escape rhythm: Retrograde atrial stimulation is caused by centers other than the sinus node. In this instance, bradycardia occurs with small QRS complexes but without P waves (the QRS complexes and P waves are synchronized).

Step 3: Electrical Heart Axis

The electrical heart axis can be determined using the Cabrera circle, which is complicated, or by examining the waves of the QRS complex (in limb leads I, II and III).

Step 6: Repolarization

Repolarization includes the ST segment and the T wave (repolarization of chambers). The standard ST segment should be an isoelectric line. Elevations and depressions of the ST segment are, therefore, pathological abnormalities, (specifically > 1 mm in the limb leads and > 2 mm in the chest leads).

Image: “Schematic diagram of normal sinus rhythm for a human heart as seen on ECG.” by Agateller (Anthony Atkielski). License: Public Domain

The most important causes for such this type of ST elevation, are acute myocardial infarction (AMI) and acute pericarditis. In cases of AMI with ST-segment elevation (STEMI), the ST-segment takes off from the descending limb of the R wave, whereas in cases of pericarditis, it takes off from the ascending limb of the S wave.

Image: “A 12-lead ECG showing a STEMI. Elevation of the ST segment can be seen in some leads.” by James Heilman, MD. License: CC BY-SA 4.0

Note: An indication for STEMI is ST-segment elevation with poor R wave progression in at least two limb leads (amplitude > 0.1 mm) or two adjacent breast leads (amplitude > 0.2 mm). ST segment depressions > 1 mm that is downsloping, horizontal or descending are considered pathological and point to acute myocardial ischemia. Downsloping depressions can also be found in digitalis therapy.

Inverted T waves: The causes for Inverted T waves vary, including acute myocardial infarction, pulmonary embolism. Therefore, these findings should always be analyzed in conjunction with the rest of the ECG, as well as the patient’s other clinical signs.

Keep in mind: Inverted T waves are not considered pathological per se. They are obligatory in lead aVR and can also be found in leads III, V1 and V2, without being a sign of disease.

Step 7: The R/S Ratio

Usually, the R wave height in the breast leads increases, while the S wave decreases and the S wave is completely missing in V6. The R/S ratio is considered to be the area where R is taller than S (usually between V2 and V3, or V3 and V4). If this is not the case, the situation is referred to as poor R wave progression. This may be an indication of myocardial infarction or left ventricular hypertrophy.

How to Interpret an ECG: An Overview

In order to be able to get a good first impression of an ECG, these seven steps are sufficient.

How to interpret an ECG in seven easy steps:

Heart frequency

Heart rhythm

Electrical heart axis

PR interval

QRS complex

Repolarization

R/S ratio

While these steps are a good start, these guidelines are not to be considered complete. Rather, these simple steps will simplify the approach to reading an ECG, which will help create a systematic interpretation of the ECG during clinical practice.

Review Question

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